This invention relates to a process for the production of non-foamed moldings using cast-resin technology.
The production of cast-resins, for example for the electrical field, by reaction of organic polyisocyanates with polyhydroxyl compounds is already known (see, for example, Kunststoff-Handbuch, Vol. VII, "Polyurethane" by Becker/Braun, Carl Hanser Verlag, Munich/Vienna (1983), pages 410 to 425). Such moldings used in the electrical industry, for example as insulators, must be bubble-free and foam-free. Consequently, the prior art teaches that careful degassing and drying of the starting materials, particularly the polyhydroxyl compounds is important. In general, water-absorbing agents such as (anhydrous) zeolites, are added to such reaction mixtures to prevent bubble formation caused by reaction of isocyanate groups with water which reaction is accompanied by the elimination of carbon dioxide.
It is also known that the incorporation of urea groups in polyurethane plastics generally leads to an improvement in their mechanical properties. Urea groups may be incorporated in polyurethanes by using either diamines or water as a reactant for the isocyanate groups. However, diamines cannot be used in the production of plastics by cast-resin technology because diamines are so highly reactive to isocyanate groups that reaction systems containing diamines cannot be processed as cast-resins. Reaction systems containing polyisocyanates, polyhydroxyl compounds and diamines cannot be processed as cast-resins in one-step because the different reactivity of the amino and hydroxyl groups to isocyanate groups would lead to inhomogeneous reaction products. The use of water, however, appeared extremely inappropriate for the reasons explained above but particularly because the presence of water would give rise to unacceptable bubble formation.
However, it has now surprisingly been found that urea groups may readily be incorporated even in bubble-free polyisocyanate-based moldings produced using cast-resin technology by using water provided the water is incorporated in the reaction mixture in the form of certain aqueous zeolites.